A core function of extra corporeal blood treatment systems (ECBT systems) such as hemodialysis, hemofiltration, hemodiafiltration, apheresis, etc. systems is the maintenance of the overall fluid balance between the fluid added to the patient and the fluid withdrawn from the patient. Ideally, this exchange will result in a net loss or gain of fluid to/from the patient that precisely matches the patient's treatment requirement. To achieve this, the ECBT may employ a volumetric fluid balancing system, of which a variety of different types are known. For example, see U.S. Pat. Nos. 5,836,908, 4,728,433, 5,344,568, 4,894,150, and 6,284,131, each of which is hereby incorporated by reference as if fully set forth in their entireties herein.
Fluid balancing mechanisms generally attempt to ensure that the total volumes of fluid pumped into and taken off the non-blood side of a filter or dialysis are equal. To provide for a desired differential between the net volume removed/added and a perfect balance, a fluid bypass driven by a separate pump may be employed. For example, this may be an ultrafiltration (“UF”) line drawing extra fluid from the waste side of a hemofilter before the waste stream enters the balancing mechanism.
It is important to maintain the fluid balance such that the amount of fluid added or removed from the patient is accurately controlled. However, all balancing systems are imperfect. In addition, many balancing systems, in order to provide ease of use and compactness, employ incremental volume, mass, or flow measurements that can accumulate error. Thus there is a perennial need in the art for higher accuracy in balancing systems.